1. Field of the Invention
The present invention relates generally to a method of and a system for testing of RF diode digital phase shifters, and particularly to an arrangement by which all phase shifting diodes and drive circuits of the phase shifter can be tested for proper operation while applying logic input signals to the phase shifter in certain patterns.
2. Description of the Known Art
Electronically controlled radio frequency (RF) phase shifters play an important role in, inter alia, microwave antenna systems of the kind in which a number of antenna elements are each driven from a common source of RF wave energy signals but at relative phases determined by the settings of individual phase shifters associated with each of the antenna elements. Such antenna systems, commonly known as phased array antennas, enable a beam of wave energy to be directed from the antenna elements at a desired angle relative to the antenna axis, even though the individual elements remain fixed in position. That is, by setting the phase shifts of the individual phase shifters associated with the antenna elements at certain initial phase shift settings, and then changing the settings of the phase shifters over successive time intervals in a predetermined pattern, it is possible to scan the main beam of wave energy radiated from the array without the need for mechanical rotation means for the antenna. An important application for phased array antennas requiring a high degree of reliability for the phase shifters incorporated with the antenna, is in microwave landing system (MLS) configurations in which at least two phased array antennas are provided adjacent an aircraft runway, for causing both an azimuth (AZ) and an elevation (EL) beam to be scanned or steered rapidly "to" and "fro", and "up" and "down". Equipment on board an aircraft approaching the runway receives the beams as scanned by the phased array antennae and, based on synchronization information obtained from a third runway antenna, calculates the aircraft heading and angle of descent relative to the runway. It is thus clear that a failure of a phase shifter such as to cause an error in the predetermined scanning operation of either the AZ or EL antenna in an MLS system, would result in the aircraft equipment deriving false positional information during the critical runway approach time of flight.
RF phase shifters are generally of two kinds, viz., diode and ferrite. The former are employed with phased array antennae used in the MLS systems and other systems in which power supplied through the phase shifter to its associated antenna element (or elements) does not exceed about ten watts. The ferrite phase shifters are used in higher power applications.
An arrangement by which the supply of a RF wave energy signal to a malfunctioning phase shifter is cut off, to avoid serious degradation of the pattern of energy radiated by a phased array antenna, is disclosed in U.S. Pat. No. 4,359,740 issued Nov. 6, 1982 to R. F. Frazita and assigned to the assignee of the present invention. The disclosure of the '740 patent is incorporated in its entirety herein by reference. As mentioned in the '740 patent, diode phase shifters are known which include built-in test equipment (BITE) to determine the presence of malfunctioning diode elements in the phase shifter. The known BITE, however, has the following disadvantages.
Diode phase shifters typically have a number (e.g., four) of logic input terminals and a correspond ing number of phase shifting networks to carry out a certain incremental relative phase shift between a radio frequency signal entering the network and the signal as it leaves the network. For example, a four-bit phase shifter includes four separately driven networks for providing incremental phase shifts of 221/2.degree., 45.degree., 90.degree. and 180.degree.. The known test circuitry allowed for testing of only one or two of the four phase shifting networks in the typical diode phase shifter and, moreover, could not detect the presence of a single open diode element in any of the networks. Test evaluations were performed only on the basis of the attainment of certain test voltages or currents, but not both.